Today, PS (presensitized) plates in which positive type photosensitive agents of which the principal components are diazo compounds and phenolic resins, or negative type photosensitive agents of which the principal components are acrylic monomers or prepolymers thereof, are being used for lithographic offset printing plates, but these plates all have low sensitivity and plate making is carried out by attaching a pre-imagewise recorded film original and exposing the plate to light. On the other hand, as a result of the progress which has been made in recent years in computer graphics processing and large capacity data storage, and with data transmission techniques, electronic editing systems in which everything from original input through correction, editing and layout to page compilation is handled by computer and can be put immediately to terminal plotters in distant locations by means of a high speed communication networks or by satellite communication have been put into practical use. The need for electronic editing systems is especially great in the newspaper printing field where immediacy is essential. Furthermore, as a result of the development of ultralarge capacity memory devices, such as optical disks, for example, consideration has been given to the storage of originals as digital data on these recording media in fields where the original is stored as a master film and a plurality of printing plates are made, as required, on the basis of the stored original.
However, there are no really practical direct type printing plates from which a printing plate can be made directly from the output of a terminal plotter, and even when an electronic editing system has been used, the output is materialized using a silver salt photographic film in practice and the printing plate is produced indirectly on the basis of this film which is attached to a PS plate prior to exposure. One of the reasons for this is the difficulty involved in the production of direct type printing plates which have a sufficiently high speed to enable the printing plate to be made in a practical period of time using the light sources used in output plotters, such as He-Ne lasers and semiconductor lasers.
Electrophotographic photosensitive materials have been considered as photosensitive materials which are highly sensitive to light and which could provide direct type printing plates. Printing plate precursors for electrophotographic plate making purposes of the type with which the photoconductive layer in the nonimage parts is removed after forming a toner image are already well known. Thus, printing plate precursors for electrophotographic plate making purposes have been disclosed, for example, in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426, JP-B-46-39405, JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656 and JP-A-57-161863 (the terms "JP-A" and "JP-B" as used herein refers to "published unexamined Japanese patent application" and "examined Japanese patent publication", respectively).
It is usually necessary to remove the nonimage parts with an alkaline etchant to expose a hydrophilic surface if an electrophotographic photosensitive material is to be used as a printing plate, and so resins which dissolve or swell in alkaline solvents and are stripped away are often used as binder resins. In general, resins which dissolve or swell in alkaline solvents have poor compatibility with organic photoconductive compounds when compared to the polycarbonate resins which are widely used as binder resins for electrophotographic materials and, consequently, the amount of organic photoconductive compound which can be introduced into the electrophotographic photosensitive layer is limited. Even if sufficient carriers for eliminating the surface potential are formed in the photoconductive layer, if the organic photoconductive compound content in the photoconductive layer is low then the rate of migration of the carriers in the photoconductive layer is reduced and the rate at which the surface potential falls is low, which is to say the response rate is low, since it is the organic photoconductive compound which has the function of transporting the carriers in the photoconductive layer. Consequently, the time elapsing from the completion of exposure before the surface potential has been reduced sufficiently so that fogging does not occur and toner development can commence is prolonged. Consideration has been given to increasing the exposure brightness and shortening the exposure time in order to shorten the overall process from surface charging to development as much as possible, but the response time is long even when the exposure time is reduced with the result that there is little shortening of the overall processing time. Hence, the slowness of the response rate greatly impedes any shortening of the overall processing time. Furthermore, other problems arise in cases where a scanning exposure is carried out using a high brightness light source, such as a laser light source, for example. Thus, if the response rate is low, the rate of reduction of the surface potential differs in the parts where write-in starts and finishes. That is to say, there is no fogging in the parts where write-in starts but fogged images are formed where write-in finishes and errors inevitably occur when a printing plate is made. Known binder resins which have been used in the past in printing plate precursors for electrophotographic plate making purposes include styrene/maleic anhydride copolymers, vinyl acetate/crotonic acid copolymers, vinyl acetate/maleic anhydride copolymers, phenolic resins, and copolymers of acrylic acid ester monomers or methacrylic acid ester monomers and monomers which contain a carboxylic acid group, as disclosed, for example, in JP-B-41-2426, JP-B-37-17162, JP-B-38-6961, JP-A-52-2437, JP-A-54-19803, JP-A-54-134632, JP-A-55-105254, JP-A-50-19509, JP-A-50-19510, JP-A-57-161863 and JP-A-58-76843. However, various problems are known to arise when these are used in printing plate precursors for electrophotographic plate making purposes in which organic photoconductive compounds are used. Thus, the film forming properties are poor and cracking occurs, and the adhesion of the film is poor and unable to withstand the printing of large numbers of copies. Furthermore, when these binders are used the response rate is generally slow and the problems which have appeared in recent years resulting from this have still to be resolved.